The basic time division multiplexed digital transmission unit is known as the DS1 signal. The basic DS1 signal format developed by a digital channel bank consists of 24 eight-bit words and one framing bit for a total of 193 bits per frame. The 24 words typically represent 24 separate and distinct messages deposited in 24 separate and distinct channels. The words are PCM (pulse code modulated) encoded and the least significant bit (i.e., eighth bit) of a channel is periodically dedicated for signaling purposes.
In some existing transmission systems the frames of the DS1 signal have been arranged into a so-called super frame comprising two groups of six frames each. Framing bits of the odd frames in the super frame are used to provide a pattern for per frame synchronization (1010 . . . pattern) and the framing bits of the even frames are used to provide a pattern for so-called subframe identification (111000111000 . . . pattern). The subframe pattern identifies the sixth and twelfth frames of the super frame in which A and B signaling bits, respectively, are inserted in the least significant bit of each channel.
Prior systems employing the DS1 signal have used various techniques for monitoring transmission performance. To this end, measurements of error indications per unit of bits have been made and when prescribed limits are exceeded, alarms are enabled thereby alerting craftpersons that the system is malfunctioning. One error indication which has been used for alarming is the violation of the bipolar characteristic of the basic DS1 signal. A major problem with using a measure of bipolar violations is that the bipolar nature of the signal is lost in the digital transmission hierarchy. That is to say, when the DS1 signal is transmitted over a transmission system higher in the transmission hierarchy, e.g., T2 or T4M, the bipolar nature of the signal is converted to unipolar and a parity check is employed to monitor performance. Consequently, the use of bipolar violations is restricted only to systems transmitting over T1 carrier repeater lines. Therefore, end-to-end performance monitoring is not possible by monitoring bipolar violations when the signal is transmitted over a system which employs unipolar transmission.
In other applications framing errors are measured and employed to monitor transmission performance. A problem with the use of framing errors is that the framing bits occur relatively infrequently and, consequently, error measurement intervals are substantially longer than those attainable when using bipolar violations.